Both utility and industrial users of induction motors and synchronous motors equipped with starting/stabilization (amortisseur) cages have historically been concerned in finding reliable procedures for detecting rotor faults or "cage faults" before such faults exacerbate to the point of catastrophic motor failure. Obviously, such motor failures can force expensive shutdowns of factory production and in certain applications can have adverse safety implications.
Until recently, the detection of rotor faults in induction motors involved taking the motor out of service, disassembly and/or the application of special instrumentation internally and/or externally of the motor. For example, U.S. Pat. No. 4,808,932 discloses a broken rotor bar detector which requires the installation of a flux coil wound on one of the stator teeth of an induction motor. Implementation of such a detector requires motor disassembly, unless the flux coil is installed at the time of manufacture or rewinding. U.S. Pat. No. 4,761,703 discloses a rotor fault detector based on motor current analysis, but requires the stationing of a flux coil at an appropriate external position proximate the induction motor to pick up stray axial flux in order to precisely determine motor speed. This approach does not require motor disassembly, or, in many cases, an interruption in service. However, in some motor applications it is not convenient from the standpoint of accessibility or there is no space available to station the flux coil in an appropriate external flux detection position. In applicants' copending application entitled "Spectral Analysis of Induction Motor Current to Detect Rotor Faults with Reduced False Alarms" Ser. No. 07/512,223 filed Apr. 20, 1990, rotor faults, such as broken bars, are detected exclusively by analysis of the motor current frequency spectrum.
To reliably detect rotor faults in accordance with the above-cited U.S. Pat. No. 4,761,703 and copending application, the induction motor being tested must be under steady state operating conditions, i.e., constant motor current and uniform loading. Unfortunately, in certain motor applications, steady state operating conditions do not exist or exist for such a short time period that reliable rotor fault detection by motor current spectral analysis is not possible. Specifically, the character of the motor service may be such that the motor operating cycle may be so short and/or the motor load so varied that a steady state operating condition is never achieved. An example of such service is induction motor driven valves widely utilized in utility power plants, wherein the complete operating cycle from start to stop may be only several seconds for the smaller valves and ten to fifteen seconds for the larger valves.
There are many occasions when it would be advantageous to rotor fault test an induction motor under no load conditions. For example, service shops would prefer to test an induction motor for cracked or broken rotor bars simply by energizing it and analyzing the motor current and/or axial flux without having to mechanically connect the motor to some sort of load or dynamometer. Unfortunately, reliable rotor fault detection pursuant to U.S. Pat. No. 4,761,703 and applicant's cited copending application requires the induction motor to be uniformly loaded during the data collection period.
Heretofore, there has been no reliable procedure for non-invasively testing the amortisseur of synchronous motors for broken bars. This is due to the fact that amortisseur bars carry appreciable current only during startup and little or no current under steady state operating conditions. Consequently, the perturbations of motor current and axial flux caused by cracked or broken amortisseur bars are too small under steady state operating conditions to provide a basis for rotor fault detection.
It is accordingly an object of the present invention to provide an improved method and apparatus for practicing same to detect rotor faults in electric motors.
A further object is to provide a method and apparatus of the above-character for detecting rotor faults in electric motors under transient operating conditions.
Another object is to provide a method and apparatus of the above-character for detecting rotor faults in electric motors under no load conditions.
An additional object is to provide a rotor fault detection method and apparatus of the above-character, which rely solely on analysis of motor current and thus are totally non-invasive of the motor operating environment.
Yet another object is to provide a rotor fault detection method and apparatus of the above-character, which are capable of detecting broken or cracked rotor bars in synchronous as well as induction motors.
Other objects of the invention will in part be obvious and in part appear hereinafter.